In my copending applications identified above, as well as in my prior U.S. Pat Nos. 3,747,010, 3,747,011 and 3,747,012, I have disclosed a system of this type wherein a contactless motion detector includes an oscillator and an amplifier connected to a load via a two-wire circuit which serves both for the actuation of that load and for the energization of the oscillator and amplifier. An output thyristor, triggerable by a detector-controlled switching transistor, is connected across all or part of a voltage-generating network that includes an electronic breakdown device, specifically a Zener diode, in series with a high-ohmic resistor which is short-circuited upon the firing of the thyristor (either in the absence or in the presence of the object whose approach is to be monitored), the voltage drop across the Zener diode insuring the availability of a sufficient operating voltage for the detector in either state of conductivity of the thyristor. My copending applications also teach the inclusion in the voltage-generating network of a storage capacitor connected in parallel with the breakdown device and separated therefrom by a decoupling diode, this capacitor serving to smooth the ripples of the raw-rectified supply voltage which insures prompt cutoff of the thyristor upon de-energization of its gate.
It has already been proposed (see German published specification No. 2,149,063) to protect a system of this general type against overloads or short circuits by inserting a current-sensing resistor in series with the output thyristor, this resistor being shunted by a charging capacitor connected to it through a diode. The charging capacitor lies in the emitter-base circuit of a transistor whose emitter-collector circuit is connected across the gate-cathode path of the output thyristor. When the sensing resistor is traversed by an abnormally high current, the voltage drop thereacross turns on the transistor and shorts the thyristor gate to its cathode. However, the short-circuiting transistor conducts only for a limited period determined by the time constant of an R/C circuit which includes the charging capacitor and the transistor base. As the base current of the saturated transistor is substantial, the capacitor eventually discharges since the monitoring current-sensing resistor -- and therefore the voltage drop across it -- must be relatively small in order to avoid untimely conduction. After the transistor has been cut off, the output thyristor can fire on the next half-cycle of the supply voltage even if the overload condition still exists. Thus, the short-circuit protection afforded by such circuitry is of limited duration.